Ribit

The issue you are raising touches upon the prohibition of Ribit (interest or usury) between Jews, which is a very serious transgression according to Jewish law (Halakha).

I was told that the "Paybox" app that allows sending money enables payments only of full shekalim. So when I use it to repay a friend, is rounding up a problem of ribbit?

The property belongs to the person and for specific reasons he needs to give it to another person.